I'm afraid that Brian Blessed no longer counts as a live actor.
There must be a role for Alan Titchmarsh, he's in everything else.
Carrot is difficult: how about Simon King, the wildlife presenter? I don't know if he can act, but he has the naive enthusiasm down pat. In which case Bill Oddie has to be found a role as one of the minor wizards.
Cate Blanchett would make the Queen of the Elves.
Ponder Stibbins is an intellectual Harry Potter, no? Problem solved. Or the actor who plays Michael in Robert Lindsay/Zoe Wannamaker's disfunctional family. Hmm. Perhaps there's a potential Magrat there, too?